1. Technical Field of the Invention
The present invention relates to the recovery of natural gas liquids, such as propane and ethane, from liquefied natural gas (LNG) streams by means of cold utilization.
2. Description of Prior Art
The supply of LNG varies considerably in composition, depending upon the source of the stream. This composition variation is particularly noticeable in the receiving terminals in the U.S. As a result of this variation, once the LNG is processed, the heating value of the revaporized LNG, or residue gas, from such processes also varies significantly. At times, the residue gas stream produced is lean without significant amounts of relatively heavier compounds, such as ethane and similar compounds. Other times, the residue gas stream can be too high in heating value for residue gas pipelines. Pipelines have specifications that have a maximum allowable heating value, or BTU content, for residue gas streams. If the heating value exceeds the maximum allowable heating value, components contributing to the high BTU content have to be removed or the pipeline operator can reject the residue gas.
Many processes have been developed to separate liquefied natural gas into a methane-rich overhead stream and a bottoms stream containing components such as C2 and heavier hydrocarbons. Examples can be found in U.S. Pat. No. 3,837,172 issued to Markbreiter (“the Markbreiter patent”), U.S. Pat. No. 5,114,451 issued to Rambo (“the Rambo patent”), and U.S. Pat. No. 6,510,706 issued to Stone et al. (“the Stone patent”).
The Markbreiter patent shows a process for separating liquefied natural gas in a fractionation column to yield a methane-enriched overhead stream and a bottoms stream that contains C2 and heavier hydrocarbons. Difficulties encountered with the process of Markbreiter include a lack of flexibility in the process to account for varying feed compositions. The Markbreiter process can result in an overhead stream, which is delivered as a vapor stream, with an unacceptably high amount of C2, which can result in rejection of the gas. Since the overhead stream is delivered in vapor form, it is necessary to compress the vapor to deliver it at the conditions of the pipeline.
The Rambo patent shows a process for separating liquefied natural gas in which a portion of a methane-enriched overhead stream from a fractionation column is re-fed to the column as reflux, while the remaining portion of the overhead stream is recovered as a vapor product stream. As with Markbreiter, additional compression steps are required to deliver the vapor at the conditions of the pipeline.
The Stone patent shows a process for removing ethane and heavier components as a liquid NGL product from a pressurized LNG stream. The Stone patent defines the pressurized LNG stream as being pressurized up to a pressure where its bubble point is equivalent to about −170° F. The Stone patent specifically describes the recovery of NGL from pressurized LNG (PLNG). A split is provided for the feed stream to provide cold reflux to the fractionation column. The cold reflux stream is pressurized so that its bubble point temperature is about −170° F., as opposed to conventional LNG processes having a bubble point about −260° F., which reduces separation efficiency within the fractionation column.
A need exists for a flexible process that will control or decrease the heating value of the residue gas produced from typical LNG processes. It would be advantageous to provide a process that will increase the amount of natural gas liquids that are recovered from LNG streams, while decreasing the heating value of the residue gas stream produced by revaporizing the lean LNG stream. It would also be advantageous to provide a process in which the product is delivered at similar conditions as the LNG feed to minimize the intervention in existing terminals and reduce the energy required to supply the gas at pipeline conditions.